1. Field of the Invention
The invention relates to a device for determining the magnitude and the direction of a relative displacement of two objects, comprising on the one hand an elongate scale which is provided on the first object and which extends in the direction of displacement, which scale consists of a succession of magnetic north poles and south poles, and on the other hand a measuring head which is provided on the second object and which comprises at least a first and a second magnetic field sensor which are arranged with respect to the scale so that under the influence of the alternating passage of north poles and south poles due to the relative displacement they are exposed to a magnetic field which alternates as a function of time and which causes an output signal in each of the sensors, the output signals of the two sensors varying in substantially the same way as a function of time but being phase-shifted with respect to one another.
2. Art Background
A device of the type referred to above is known from GB-A 1,492,980. Such devices can be used, for example for measuring linear displacements in which case the scale extends along a straight line, or for measuring the speed of revolution and direction of a wheel, in which case the scale extends along the circumference of the wheel. As is described with reference to FIG. 3 of GB-A 1,492,980, the extent and the direction of the relative displacement can be simply determined from the phase-shifted output signals of the sensors. In order to ensure that the signals of the two sensors are phase-shifted, in the known device the sensors are arranged so that the second sensor has been shifted with respect to the first sensor over an accurately defined distance, depending on .lambda., in the longitudinal direction of the scale, where .lambda. is the distance between two homonymous poles of the scale. Therefore, the magnetic poles must be provided at exactly the same distances from one another on the scale and the distance between the two sensors must be accurately adjusted and exactly adapted to .lambda.. Consequently, the manufacture of the known device is complex and expensive. It is a further drawback of the known measuring head that its dimension in the longitudinal direction of the scale is comparatively large.